1. Field of the Invention
The present invention relates to a pneumatic tire provided with a plurality of sipes in the tread portion. More particularly, the present invention relates to a pneumatic tire having improved performance on icy and wet road surfaces without degradation of steering stability on dry road surfaces.
2. Background Information
In pneumatic tires for winter, such as studless tires, a plurality of circumferential grooves that extend in the circumferential direction and a plurality of lateral grooves that extend in the tire width direction are formed in the tread portion. A plurality of blocks is partitioned by these circumferential grooves and lateral grooves, and a plurality of sipes is formed in each block extending in the tire width direction. The method of improving the performance on ice by removing the water film on the surface of the ice by providing the plurality of sipes in the tread portion in this manner is known. Normally, if the density of the sipes is increased by increasing the number of sipes in order to further improve the performance on ice, the block rigidity is reduced, and the steering stability on dry road surfaces tends to reduce. Therefore, it has been proposed that raised bottom portions be provided in the ends of the sipes to minimize the reduction in block rigidity as described, for example, in Japanese Unexamined Patent Application Publication No. 2010-274846.
However, if raised bottom portions are provided in places such as the ends of the sipes, the communicating portion of the sipes with the circumferential grooves become smaller. Therefore, the effect of removal of the water film is reduced, and thus, the tire may not exhibit both good performance on icy road surfaces and good performance on wet road surfaces.